Electrically switchable polymer dispersed liquid crystal materials including transmissive holographic gratings

ABSTRACT

A new photopolymerizable material allows single-step, fast recording of volume holograms with properties that can be electrically controlled. Polymer-dispersed liquid crystals (PDLCs) in accordance with the invention preferably comprise a homogeneous mixture of a nematic liquid crystal and a multifunctional pentaacrylate monomer, in combination with photoinitiator, coinitiator and cross-linking agent. Optionally, a surfactant such as octanoic acid may also be added. The PDLC material is exposed to coherent light to produce an interference pattern inside, the material. Photopolymerization of the new PDLC material produces a hologram of clearly separated liquid crystal domains and cured polymer domains. Volume transmission gratings made with the new PDLC material can be electrically switched between nearly 100% diffraction efficiency and nearly 0% diffraction efficiency. By increasing the frequency of the switching voltage, switching voltages in the range of 50 Vrms can be achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/034,014 filed on Mar. 2, 1998 which is a continuation of U.S. patentapplication Ser. No. 08/680,292, filed Jul. 12, 1996, which issued asU.S. Pat. No. 5,942,157, both of which are incorporated herein byreference in their entireties.

FIELD OF INVENTION

The present invention relates generally to photopolymerizable materials,and more specifically to polymer-dispersed liquid crystal materialssuitable for recording volume holograms.

BACKGROUND OF THE INVENTION

Typical state-of-the-art holographic materials do not have anelectro-optical nature which can be exploited for real time control oftheir optical properties. That is, once the hologram is fixed, itsoptical characteristics cannot be changed. Thus, it is seen that thereis a need for materials that can record volume holograms with propertiesthat can be electrically controlled.

Liquid crystals have long been utilized in the prior art for theirability to change their optical orientation in the presence of anelectric field. Additionally, liquid crystals can dramatically increasethe diffraction efficiency of a volume hologram of which they are apart. Together, these properties offer the very desirable possibility ofelectrically switching the diffraction efficiency of volume hologramsfor use in a wide variety of optical information processing and displayapplications.

The prior art has attempted to combine the properties of liquid crystalswith holograms by a variety of methods. Unfortunately, most of theseprior art devices are complex to manufacture and are not successful atoffering all the advantages of volume holographic gratings.

One approach for combining the advantages of liquid crystals with volumeholographic gratings has been to first make a holographic transmissiongrating by exposing a photopolymerizable material with a conventionaltwo-beam apparatus for forming interference patterns inside thematerial. After exposure, the material is processed to produce voidswhere the greatest amount of exposure occurred, that is, along thegrating lines, and then, in a further step, the pores are infused withliquid crystals. Unfortunately, these materials are complex tomanufacture and do not offer flexibility for in situ control over liquidcrystal domain size, shape, density, or ordering.

Polymer-dispersed liquid crystals (PDLCS) are formed from a homogeneousmixture of prepolymer and liquid crystals. As the polymer cures, theliquid crystals separate out as a distinct microdroplet phase. If thepolymer is a photopolymer, this phase separation occurs as theprepolymer is irradiated with light. If a photopolymerizablepolymer-dispersed liquid crystal material is irradiated with light in asuitable pattern, a holographic transmission grating can be made insidethe cured polymer comprising gratings of cured polymer separated byphase separated liquid-crystals. The prior art has attempted to employpolymer-dispersed liquid crystal materials for writing volume gratings,but, despite a variety of approaches, has not been able to achieve highefficiency in the Bragg regime, high density (small grating spacing)capability, or low voltage (<100 Vrms) switching for films in the rangeof 15 microns thickness. The inability to make an electricallyswitchable volume hologram that can be switched at voltages less than100 volts has been a particular deficiency in the prior art in thatlower voltages are necessary to be compatible with conventional displayand information processing technology.

OBJECTS OF THE INVENTION

It is, therefore, a principal object of the present invention to providean improved polymer-dispersed liquid crystal system suitable forrecording volume holograms.

It is a particular object of the present invention to provide apolymer-dispersed liquid crystal system that has a fast curing rate toproduce small liquid crystal droplets, particularly in the range of0.01-0.05 microns, for greater clarity of any resulting film and forwriting finer gratings.

It is another object of the present invention to provide a single-step,fast holographic recording material.

It is a further object of the present invention to provide electricallyswitchable volume holograms that can be switched at voltages less than100 volts.

It is also an object of the present invention to provide an improvedpolymer-dispersed liquid crystal system suitable for recordingreflection gratings, including, in particular, switchable reflectiongratings.

It is also an object of the present invention to provide an improvedpolymer-dispersed liquid crystal system suitable for recordingsubwavelength gratings, including, in particular switchablesubwavelength gratings.

These and other objects of the present invention will become apparent asthe description of certain representative embodiments proceeds.

SUMMARY OF THE INVENTION

The present invention provides a novel photopolymerizable material forsingle-step, fast recording of volume holograms with properties that canbe electrically controlled. The unique discovery of the presentinvention is a new homogeneous mixture of a nematic liquid crystal and amultifunctional pentaacrylate monomer, with a photoinitiator, is acoinitiator and a cross-linking agent, that accomplishes the objects ofthe invention, particularly the object of fast curing speed and smallliquid crystal droplet size.

Accordingly, the present invention is directed to a polymer-dispersedliquid crystal (“PDLC”) material, comprising the monomerdipentaerythritol hydroxypentaacrylate, a liquid crystal, across-linking monomer, a coinitiator and a photoinitiator dye. Thepolymer-dispersed liquid crystal material may optionally furthercomprise a surfactant. The PDLC material may be approximately 10-40 wt %of the liquid crystal. The PDLC material may be approximately 5-15 wt %of the cross-linking monomer. The amount of the coinitiator may be 10⁻³to 10⁻⁴ gram moles and the amount of the photoinitiator dye may be 10⁻⁵to 10⁻⁶ gram moles. The surfactant, when present, may be up toapproximately 6 wt % of the PDLC material.

The present invention is also directed to an electrically switchablehologram, comprising a pair of transparent plates, and sandwichedbetween the transparent plates, a volume hologram made by exposing aninterference pattern inside a polymer-dispersed liquid crystal material,the polymer-dispersed liquid crystal material comprising, beforeexposure, the monomer dipentaerythritol hydroxypentaacrylate, a liquidcrystal, a cross-linking monomer, a coinitiator and a photoinitiatordye. The electrically switchable hologram may optionally furthercomprise a surfactant.

The present invention is additionally directed to a method for reducingthe switching voltage needed to switch the optical orientation of liquidcrystals in a polymer-dispersed liquid crystal material, comprising thestep of using alternating current switching voltage frequencies greaterthan 1000 Hz.

It is a feature of the present invention that a very clear and orderlyseparation of liquid crystal from cured polymer results, so as toproduce high quality holographic transmission gratings. The prior arthas achieved generally only a distribution of large and small liquidcrystal domains and not the clear, orderly separation made possible bythe present invention.

It is also a feature of the present invention that volume Bragg gratingswith small grating spacings (approximately 4,000 lines per mm) can berecorded.

It is another feature of the present invention that in situcontrol ofdomain size, shape, density, and ordering is allowed.

It is yet another feature of the present invention that holograms can berecorded using conventional optical equipment and techniques.

It is a further feature of the present invention that a uniquephotopolymerizable prepolymer material is employed. This unique materialcan be used to record holograms in a single step.

It is also a feature of the present invention that the PDLC material hasan anisotropic spatial distribution of phase-separated liquid crystaldroplets within a photochemically-cured polymer matrix.

It is an advantage of the present invention that single-step recordingis nearly immediate and requires no later development or furtherprocessing.

It is another advantage of the present invention that uses thereof arenot limited to transmission gratings, but can be extended to otherholograms such as optical storage devices and reflection andtransmission pictorial holograms.

It is also an advantage that, unlike holograms made with conventionalphotograph-type films or dichromated gels, holograms in accordance withthe present invention can be exposed in a one-step process that requireslittle or no further processing.

It is a further advantage of the present invention that reflection,transmission and pictorial holograms made using the teachings providedherein can be switched on and off.

It is also an advantage of the present invention that switchablereflection gratings can be formed using positive and negative dielectricanisotropy liquid crystals.

These and other features and advantages of the present invention willbecome apparent as the description of certain representative embodimentsproceeds.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will be more clearly understood from a reading ofthe following detailed description in conjunction with the accompanyingfigures wherein:

FIG. 1 is a cross-sectional view of an electrically switchable hologrammade of an exposed polymer-dispersed liquid crystal material accordingto the teachings of the present invention;

FIG. 2 is a graph of the normalized net transmittance and normalized netdiffraction efficiency of a hologram made according to the teachings ofthe present invention (without the addition of a surfactant) versus therms voltage applied across the hologram;

FIG. 3 is a graph of both the threshold and complete switching rmsvoltages needed for switching a hologram made according to the teachingsof the present invention to minimum diffraction efficiency versus thefrequency of the rms voltage;

FIG. 4 is a graph of the normalized diffraction efficiency as a functionof the applied electric field for a PDLC material formed with 34% byweight liquid crystal without surfactant present and a PDLC materialformed with 29% by weight liquid crystal and 4% by weight surfactant;

FIG. 5 is a graph showing the switching response time data for thediffracted beam in the surfactant-containing PDLC material in FIG. 5;

FIG. 6 is a graph of the normalized net transmittance and the normalizednet diffraction efficiency of a hologram made according to the teachingsof the present invention versus temperature;

FIG. 7 is an elevational view of a typical experimental arrangement forrecording reflection gratings;

FIGS. 8a and 8 b are elevational views of a reflection grating inaccordance with the present invention having periodic planes of polymerchannels and PDLC channels disposed parallel to the front surface in theabsence of a field (FIG. 8a) and with an electric field applied (FIG.8b) wherein the liquid-crystal utilized in the formation of the gratinghas a positive dielectric anisotropy;

FIGS. 9a and 9 b are elevational views of a reflection grating inaccordance with the invention having periodic planes of polymer channelsand PDLC channels disposed parallel to the front surface of the gratingin the absence of an electric field (FIG. 9a) and with an electric fieldapplied (FIG. 9b) wherein the liquid crystal utilized in the formationof the grating has a negative dielectric anisotropy;

FIG. 10a is an elevational view of a reflection grating in accordancewith the invention disposed within a magnetic field generated byHelmholtz coils;

FIGS. 10b and 10 c are elevational views of the reflection grating ofFIG. 10a in the absence of an electric field (FIG. 10b) and with anelectric field applied (FIG. 10c);

FIGS. 11a and 11 b are representative side views of a slantedtransmission grating (FIG. 11a) and a slanted reflection grating (FIG.11b) showing the orientation of the grating vector G of the periodicplanes of polymer channels and PDLC channels;

FIG. 12 is an elevational view of a reflection grating formed inaccordance with the invention while a shear stress field is applied;

FIG. 13 is an elevational view of a subwavelength grating in accordancewith the present invention having periodic planes of polymer channelsand PDLC channels disposed perpendicular to the front surface of thegrating;

FIG. 14a is an elevational view of a switchable subwavelength grating inaccordance with the present invention wherein the subwavelength gratingfunctions as a half wave plate whereby the polarization of the incidentradiation is rotated by 90°;

FIG. 14b is an elevational view of the switchable half wave plate shownin FIG. 14a disposed between crossed polarizers whereby the incidentlight is transmitted;

FIGS. 14c and 14 d are side views of the switchable half wave plate andcrossed polarizers shown in FIG. 14b showing the effect of theapplication of a voltage to the plate whereby the polarization of thelight is no longer rotated and thus blocked by the second polarizer;

FIG. 15a is a side view of a switchable subwavelength grating inaccordance with the invention wherein the subwavelength gratingfunctions as a quarter wave plate whereby plane polarized light istransmitted through the subwavelength grating, retroreflected by amirror and reflected by the beam splitter;

FIG. 15b is a side view of the switchable subwavelength grating of FIG.15a showing the effect of the application of a voltage to the platewhereby the polarization of the light is no longer modified, therebypermitting the reflected light to pass through the beam splitter;

FIGS. 16a and 16 b are elevational views of a subwavelength grating inaccordance with the present s invention having periodic planes ofpolymer channels and PDLC channels disposed perpendicular to the frontface of the grating in the absence of an electric field (FIG. 16a) andwith an electric field applied (FIG. 16b) wherein the liquid crystalutilized in formation of the grating has a positive dielectricanisotropy; and

FIG. 17 is a side view of five subwavelength gratings wherein thegratings are stacked and connected electrically in parallel therebyreducing the switching voltage of the subwavelength grating.

DETAILED DESCRIPTION

In accordance with present invention there is provided a polymerdispersed liquid crystal (“PDLC”) material comprising a monomer, adispersed liquid crystal, a cross-linking monomer, a coinitiator and aphotoinitiator dye. These PDLC materials exhibit clear and orderlyseparation of the liquid crystal and cured polymer, whereby the PDLCmaterial advantageously provides high quality holographic gratings. ThePDLC materials of the present invention are also advantageously formedin a single step. The present invention also utilizes a uniquephotopolymerizable prepolymer material that permits in situ control overcharacteristics of the resulting gratings, such as domain size, shape,density, ordering, and the like. Furthermore, methods and materials ofthe present invention can be used to prepare PDLC materials thatfunction as switchable transmission or reflection gratings.

Polymer dispersed liquid crystal materials, methods, and devicescontemplated for use in the practice of the present invention are alsodescribed in R. L. Sutherland et al., “Bragg Gratings in an AcrylatePolymer Consisting of Periodic Polymer-Dispersed Liquid-Crystal Planes,”Chemistry of Materials, No. 5, pp. 1533-1538 (1993); in R. L.,Sutherland et al., “Electrically switchable volume gratings inpolymer-dispersed liquid crystals,” Applied Physics Letters, Vol. 64,No. 9, pp. 1074-1076 (1984); and T. J. Bunning et al., “The Morphologyand Performance of Holographic Transmission Gratings Recorded in PolymerDispersed Liquid Crystals,” Polymer, Vol. 36, No. 14, pp. 2699-2708(1995), all of which are fully incorporated by reference into thisDetailed Description. Copending patent applications Ser. Nos. 08/273,436now abandoned, and 08/273,437, U.S. Pat. No. 5,698,343, Sutherland etal., titled “Switchable Volume Hologram Materials and Devices,” and“Laser Wavelength Detection and Energy Dosimetry Badge,” respectively,also incorporated herein by reference include background material on theformation of transmission gratings inside volume holograms.

The process by which a hologram is formed according to the invention iscontrolled primarily by the choice of components used to prepare thehomogeneous starting mixture, and to a lesser extent by the intensity ofthe incident light pattern. The preferred polymer-dispersed liquidcrystal (“PDLC”) material employed in the practice of the presentinvention creates a switchable hologram in a single step. A new featureof the preferred PDLC material is that illumination by an inhomogeneous,coherent light pattern initiates a patterned, anisotropic diffusion (orcounter diffusion) of polymerizable monomer and second phase material,particularly liquid crystal (“LC”) for this application. Thus,alternating well-defined channels of second phase-rich material,separated by well-defined channels of nearly pure polymer, are producedin a single-step process.

The resulting preferred PDLC material has an anisotropic spatialdistribution of phase-separated LC droplets within the photochemicallycured polymer matrix. Prior art PDLC materials made by a single-stepprocess can achieve at best only regions of larger LC bubbles andsmaller LC bubbles in a polymer matrix. The large bubble sizes arehighly scattering which produces a hazy appearance and multiple orderdiffractions, in contrast to the well-defined first order diffractionand zero order diffraction made possible by the small LC bubbles of thepreferred PDLC material in well-defined channels of LC-rich material.Reasonably well-defined alternately LC-rich channels and nearly purepolymer channels in a PDLC material are possible by multi-stepprocesses, but such processes do not achieve the precise morphologycontrol over LC droplet size and distribution of sizes and widths of thepolymer and LC-rich channels made possible by the preferred PDLCmaterial.

The sample is prepared by coating the mixture between twoindium-tin-oxide (ITO) coated glass slides separated by spacers ofnominally 10-20 μm thickness. The sample is placed in a conventionalholographic recording-setup. Gratings are typically recorded using the488 nm line of an Argon ion laser with intensities of between about0.1-100 mW/cm² and typical exposure times of 30-120 seconds. The anglebetween the two beams is varied to vary the spacing of the intensitypeaks, and hence the resulting grating spacing of the hologram.Photopolymerization is induced by the optical intensity pattern. A moredetailed discussion of exemplary recording apparatus can be found in R.L. Sutherland, et al., “Switchable holograms in new photopolymer-liquidcrystal composite materials,” a Society of Photo-Optical InstrumentationEngineers (SPIE), Proceedings Reprint, Volume 2404, reprinted fromDiffractive and Holographic Optics Technology II (1995), incorporatedherein by reference.

The features of the PDLC material are influenced by the components usedin the preparation of the homogeneous starting mixture and, to a lesserextent, by the intensity of the incident light pattern. In the preferredembodiment, the prepolymer material comprises a mixture of aphotopolymerizable monomer, a second phase material, a photoinitiatordye, a coinitiator, a chain extender (or cross-linker), and, optionally,a surfactant.

In the preferred embodiment, the two major components of the prepolymermixture are the polymerizable monomer and the second phase material,which are preferably completely miscible. Highly functionalized monomersare preferred because they form densely cross-linked networks whichshrink to some extent and tend to squeeze out the second phase material.As a result, the second phase material is moved anisotropically out ofthe polymer region and, thereby, separated into well-definedpolymer-poor, second phase-rich regions or domains. Highlyfunctionalized monomers are also preferred because the extensivecross-linking associated with such monomers yields fast kinetics,allowing the hologram to form relatively quickly, whereby the secondphase material will exist in domains of less than approximately 0.1 μm.

Highly functionalized monomers, however, are relatively viscous. As aresult, these monomers do not tend to mix well with other materials, andthey are difficult to spread into thin films. Accordingly, it ispreferable to utilize a mixture of penta-acrylates in combination withdi-, tri-, and/or tetra-acrylates in order to optimize both thefunctionality and viscosity of the prepolymer material. Suitableacrylates, such as triethyleneglycol diacrylate, trimethylolpropanetriacrylate, pentaerythritol triacrylate, pentaerythritol tetracrylate,pentaerythritol pentacrylate, and the like can be used in, accordancewith the present invention. In the preferred embodiment, it has beenfound that an approximately 1:4 mixture of tri- to penta-acrylatefacilitates homogeneous mixing while providing a favorable mixture forforming 10-20 μm films on the optical plates.

The second phase material of choice for use in the practice of thepresent invention is a liquid crystal. This also allows anelectro-optical response for the resulting hologram. The concentrationof LC employed should be large enough to allow a significant phaseseparation to occur in the cured sample, but not so large as to make thesample opaque or very hazy. Below about 20% by weight very little phaseseparation occurs and diffraction efficiencies are low. Above about 35%by weight, the sample becomes highly scattering, reducing bothdiffraction efficiency and transmission. Samples fabricated withapproximately 25% by weight typically yield good diffraction efficiencyand optical clarity. In prepolymer mixtures utilizing a surfactant, theconcentration of LC may be increased to 35% by weight without loss inoptical performance by adjusting the quantity of surfactant. Suitableliquid crystals contemplated for use in the practice of the presentinvention include the mixture of cyanobiphenyls marketed as E7 by Merck,4′-n-pentyl-4-cyanobiphenyl, 4′-n-heptyl-4-cyanobiphenyl,4′-octaoxy-4-cyano biphenyl, 4′-pentyl 4-cyanoterphenyl,4-methoxybenzylidene-4′-butylaniline, and the like. Other second phasecomponents are also possible.

The preferred polymer-dispersed liquid crystal material employed in thepractice of the present invention is formed from a prepolymer materialthat is a homogeneous mixture of a polymerizable monomer comprisingdipentaerythritol hydroxypentacrylate (available, for example, fromPolysciences, Inc., Warrington, Pa.), approximately 10-40 wt % of theliquid crystal E7 (which is a mixture of cyanobiphenyls marketed as E7by Merck and also available from BDH Chemicals, Ltd., London, England),the chain-extending monomer N-vinylpryrrolidone (“NVP”) (available fromthe Aldrich Chemical Company, Milwaukee, Wis.), coinitiatorN-phenylgylycine (“NPG”) (also available from the Aldrich ChemicalCompany, Milwaukee, Wis.), and the photoinitiator dye rose bengal ester;(2,4,5,7-tetraiodo-3′,4′,5′,6′-tetrachlorofluroescein-6-acetate ester)marketed as RBAX by Spectragraph, Ltd., Maumee, Ohio). Rose bengal isalso available as rose bengal sodium salt (which must be esterfied forsolubility) from the Aldrich Chemical Company. This system has a veryfast curing speed which results in the formation of small liquid crystalmicro-droplets.

The mixture of liquid crystal and p-epolymer material are homogenized toa viscous solution by suitable means (e.g., ultrasonification) andspread between indium-tin-oxide (“ITO”) coated glass slides with spacersof nominally 15-100 μm thickness and, preferably, 10-20 μm thickness.The ITO is electrically conductive and serves as an opticallytransparent electrode. Preparation, mixing and transfer of theprepolymer material onto the glass slides are preferably done in thedark as the mixture is extremely sensitive to light.

The sensitivity of the prepolymer materials to light intensity isdependent on the photoinitiator dye and its is concentration. A higherdye concentration leads to a higher sensitivity. In most cases, however,the solubility of the photoinitiator dye limits the concentration of thedye and, thus, the sensitivity of the prepolymer material. Nevertheless,it has been found that for more general applications photoinitiator dyeconcentrations in the range of 0.2-0.4% by weight are sufficient toachieve desirable sensitivities and allow for a complete bleaching ofthe dye in the recording process, resulting in colorless final samples.Photoinitiator dyes that are useful in generating PDLC materials inaccordance with the present invention are rose bengal ester(2,4,5,7-tetra-iodo-3′,4′,5′,6′-tetrachlorofluroescein-6-acetate ester);rose bengal sodium salt; eosin; eosin sodium salt; 4,5-diiodosuccinylfluorescein; camphorquinone; methylene blue, and the like. These dyesallow a sensitivity to recording wavelengths across the visible spectrumfrom nominally 400 nm to 700 nm. Suitable near-infrared dyes, such ascationic cyanine dyes with trialkylborate anions having absorption from600-900 nm as well as merocyanine dyes derived from spiropyran shouldalso find utility in connection with the present invention.

The coinitiator employed in the practice of the present inventioncontrols the rate of curing in the free radical polymerization reactionof the prepolymer-material. Optimum phase separation and, thus, optimumdiffraction efficiency in the resulting PDLC material, are a function ofcuring rate. It has been found that favorable results can be achievedutilizing coinitiator in the range of 2-3% by weight. Suitablecoinitiators include N-phenyl glycine; triethylene amine;triethanolamine; N,N-dimethyl-2,6-diisopropyl aniline, and the like.

Other suitable dyes and dye coinitiator combinations that should besuitable for use in the present invention, particularly for visiblelight, include eosin and triethanolamine; camphorquinone andN-phenyglycine; fluorescein and triethanolamine; methylene blue andtriethanolamine or N-phenylglycine; erythrosin B and triethanolamine;indolinocarbocyanine and triphenyl borate; iodobenzospiropyran andtriethylamine, and the like.

The chain extender (or cross linker) employed in the practice of thepresent invention helps to increase the solubility of the components inthe prepolymer material as well as increase the speed of polymerization.The chain extender is preferably a smaller vinyl monomer as comparedwith the pentacrylate, whereby it can react with the aerylate positionsin the pentaacrylate monomer, which are not easily accessible toneighboring pentaacrylate monomers due to steric hindrance. Thus,reaction of the chain extender monomer with the polymer increases thepropagation length of the growing polymer and results in high molecularweights. It has been found that chain extender in general applicationsin the range of 10-18% by weight maximizes the performance in terms ofdiffraction efficiency. In the preferred embodiment, it is expected thatsuitable chain extenders can be selected from the following: N-vinylpyrralidone; N-vinyl pyridine; acrylonitrile; N vinyl carbazole, and thelike.

It has been found that the addition of a surfactant material, namely,octanoic acid, in the prepolymer material lowers the switching voltageand also improves the diffraction efficiency. In particular, theswitching voltage for PDLC materials containing a surfactant aresignificantly lower than those of a PDLC material made without thesurfactant. While not wishing to be bound by any particular theory, itis believed that these results may be attributed to the weakening of theanchoring forces between the polymer and the phase-separated LCdroplets. SEM studies have shown that droplet sizes in PDLC materialsincluding surfactants are reduced to the range of 30-50 nm and thedistribution is more homogeneous. Random scattering in such materials isreduced due to the dominance of smaller droplets, thereby increasing thediffraction efficiency. Thus, it is believed that the shape of thedroplets becomes more spherical in the presence of surfactant, therebycontributing to the decrease in switching voltage.

For more general applications, it has been found that samples with aslow as 5% by weight of surfactant exhibit a significant reduction inswitching voltage. It has also been found that, when optimizing for lowswitching voltages, the concentration of surfactant may vary up to about10% by weight (mostly dependent on LC concentration) after which thereis a large decrease in diffraction efficiency, as well as an increase inswitching voltage (possibly due to a reduction in total phase separationof LC). Suitable surfactants include octanoic acid; heptanoic-acid;hexanoic acid; dodecanoic acid; decanoic acid, and the like.

In samples utilizing octanoic acid as the surfactant, it has beenobserved that the conductivity of the sample is high, presumably owingto the presence of the free carboxyl (COOH) group in the octanoic acid.As a result, the sample increases in temperature when a high frequency(˜2 KHz) electrical field is applied for prolonged periods of time.Thus, it is desirable to reduce the high conductivity introduced by thesurfactant, without sacrificing the high diffraction efficiency and thelow switching voltages. It has been found that suitable electricallyswitchable gratings can be formed from a polymerizable monomer, vinylneononanoate (“VN”) C₈H₁₇CO₂CH═CH₂, commercially available from theAldrich Chemical Co. in Milwaukee, Wis. Favorable results have also beenobtained where the chain extender N-vinyl pyrrolidone (“NVP”) and thesurfactant octanoic acid are replaced by 6.5% by weight VN. VN also actsas a chain extender due to the presence of the reactive acrylate monomergroup. In these variations, high optical quality samples were obtainedwith about 70% diffraction efficiency, and the resulting gratings couldbe electrically switched by an applied field of 6V/μm.

PDLC materials in accordance with the present invention may also beformed using a liquid crystalline bifunctional acrylate as the monomer(“LC monomer”). The LC monomers have an advantage over conventionalacrylate monomers due to their high compatibility with the low molecularweight nematic LC materials, thereby facilitating formation of highconcentrations of low molecular weight LC and yielding a sample withhigh optical quality. The presence of higher. concentrations of lowmolecular weight LCs in the PDLC material greatly lowers the switchingvoltages (e.g., to ˜2V/μm). Another advantage of using LC monomers isthat it is possible to apply low AC or DC fields while recordingholograms to pre-align the host LC monomers and low molecular weight LCso that a desired orientation and configuration of the nematic directorscan be obtained in the LC droplets. The chemical formulae of severalsuitable LC monomers are as follows:

CH₂═CH—COO—(_(CH2))₆O—C₆H₅—COO—CH═CH₂

CH₂═CH—(CH₂)₈—COO—C₆H₅—COO—(CH₂)₈—CH═CH₂

H(CF₂)₁₀CH₂O—CH₂—C(═CH₂)—COO—(CH₂CH₂O)₃CH₂CH₂O—COO—CH₂—C(═CH₂)—CH₂O(CF₂)₁₀H

Semifluorinated polymers are known to show weaker anchoring propertiesand also significantly reduced switching fields. Thus, it is believedthat semifluorinated acrylate monomers which are bifunctional and liquidcrystalline will find suitable application in the present invention.

Referring now to FIG. 1 of the drawings, there is shown across-sectional view of an electrically switchable hologram 10 made ofan exposed polymer-dispersed liquid crystal material according to theteachings of the present invention. A layer 12 of the polymer-dispersedliquid crystal material is sandwiched between a pair of indium-tin-oxide(ITO) coated glass slides 14 and spacers 16. The interior of hologram 10shows the Bragg transmission gratings 16 formed when layer 12 wasexposed to an interference pattern from two intersecting beams ofcoherent laser light. The exposure times and intensities can be varieddepending on the diffraction efficiency and liquid crystal domain sizedesired. Liquid crystal domain size can be controlled by varying theconcentrations of photoinitiator, coinitiator and chain-extending (orcross-linking) agent. The orientation of the nematic directors can becontrolled while the gratings are being recorded by application of anexternal electric field across the ITO electrodes.

The scanning electron micrograph shown in FIG. 2 of the referencedApplied Physics Letters article and incorporated herein by reference isof the surface of a grating which was recorded in a sample with a 36 wt% loading of liquid crystal using the 488 nm line of an argon ion laserat an intensity of 95 mW/cm². The size of the liquid crystal domains isabout 0.2 μm and the grating spacing is about 0.54 μm. This sample,which is approximately 20 μm thick, diffracts light in the Bragg regime.

FIG. 2 is a graph of the normalized net transmittance and normalized netdiffraction efficiency of a hologram made according to the teachings ofthe present invention-versus the root mean square voltage (“Vrms”)applied across the hologram. Δη is the change in first order Braggdiffraction efficiency. ΔT is the change in zero order transmittance.FIG. 2 shows that energy is transferred from the first order beam to thezero-order beam as the voltage is increased. There is a true minimum ofthe diffraction efficiency at approximately 225 Vrms. The peakdiffraction efficiency can approach 100%, depending on the wavelengthand polarization of the probe beam, by appropriate adjustment of thesample thickness. The minimum diffraction efficiencycan be made toapproach 0% by slight adjustment of the parameters of the PDLC materialto force the refractive index of the cured polymer to be equal to theordinary refractive index of the liquid crystal.

By increasing the frequency of the applied voltage, the switchingvoltage for minimum diffraction efficiency can be decreasedsignificantly. This is illustrated in FIG. 3, which is a graph of boththe threshold rms voltage 20 and the complete switching rms voltage 22needed for switching a hologram made according to the teachings of thepresent invention to minimum diffraction efficiency versus the frequencyof the rms voltage. The threshold and complete switching rms voltagesare reduced to 20 Vrms and 60 Vrms, respectively, at 10 kHz. Lowervalues are expected at even higher frequencies.

Smaller liquid crystal droplet sizes have the problem that it takes highswitching voltages to switch their orientation. As described in theprevious paragraph, using alternating current switching voltages at highfrequencies helps reduce the needed switching voltage. As demonstratedin FIG. 4, another unique discovery of the present invention is thatadding a surfactant (e.g., octanoic acid) to the prepolymer material inamounts of about 4%-6% by weight of the total mixture resulted in sampleholograms with switching voltages near 50 Vrms at lower frequencies of1-2 kHz. As shown in FIG. 5, it has also been found that the use of thesurfactant with the associated reduction in droplet size, reduces theswitching time of the PDLC materials. Thus, samples made with surfactantcan be switched on the order of 25-44 microseconds. Without wishing tobe bound by any theory, the surfactant is believed to reduce switchingvoltages by reducing the anchoring of the liquid crystals at theinterface between liquid crystal and cured polymer

Thermal control of diffraction efficiency is illustrated in FIG. 6. FIG.6 is a graph of the normalized net transmittance and normalized netdiffraction efficiency of a hologram made according to the teachings ofthe present invention versus temperature.

The polymer-dispersed liquid crystal materials described hereinsuccessfully demonstrate the utility for recording volume holograms of aparticular composition for such polymer-dispersed liquid crystalsystems. Although the disclosed polymer-dispersed liquid crystal systemsare specialized, the present invention will find application in otherareas where a fast curing polymer and a material that can bephase-separated from the polymer will find use.

As shown in FIG. 7, a PDLC reflection grating is prepared by placingseveral drops of the mixture of prepolymer material 112 on an indium-tinoxide (“ITO”)-coated glass slide 114 a. A second indium-tin oxide(“ITO”) coated slide 114 b is then pressed against the first, therebycausing the prepolymer material 112 to fill the region between theslides 114 a and 114 b. Preferably, the separation of the slides ismaintained at approximately 20 μm by utilizing uniform spacers 118.Preparation, mixing and transfer of the prepolymer material ispreferably done in the dark. Once assembled, a mirror 116 is placeddirectly behind the glass plate 114 b. The distance of the mirror fromthe sample is preferably substantially shorter than the coherence lengthof the laser. The PDLC material is preferably exposed to the 488 nm lineof an argon-ion laser, expanded to fill the entire plane of the glassplate, with an intensity of approximately 0.1-100 mWatts/cm₂ withtypical exposure times of 30-120 seconds. Constructive and destructiveinterference within the expanded beam establishes a periodic intensityprofile through the thickness of the film.

In the preferred embodiment, the prepolymer material utilized to make areflection grating comprises a monomer, a liquid crystal, across-linking monomer, a coinitiator, and a photoinitiator dye. In thepreferred embodiment, the reflection grating is formed from prepolymermaterial comprising by total weight of the monomer dipentaerythritolhydroxypentacrylate (“DPHA”), 34% by total weight of a liquid crystalcomprising a mixture of cyano biphenyls (known commercially as “E7”),10% by total weight of a cross-linkingmonomer comprisingN-vinylpyrrolidone (“NVP”), 2.5% by weight of the coinitiatorN-phenylglycine (“NPG”), and 10⁻⁵ to 10⁻⁶ gram moles of a photoinitiatordye comprising rose bengal ester. Further, as with transmissiongratings, the addition of surfactants is expected to facilitate the sameadvantageous properties discussed above in connection with transmissiongratings. It is also expected that similar ranges and variation ofprepolymer starting materials will find ready application in theformation of suitable reflection gratings.

It has been determined by low voltage, high resolution scanning electronmicroscopy (“LVHRSEM”) that the resulting material comprises a finegrating with a periodicity of 165 nm with the grating vectorperpendicular to the plane of the surface. Thus, as shown schematicallyin FIG. 8a, grating 130 includes periodic planes of polymer channels 130a and PDLC channels 130 b which run parallel to the front surface 134.The grating spacing associated with these periodic planes remainsrelatively constant throughout the full thickness of the sample from theair/film to the film/substrate interface.

Although interference is used to prepare both transmission andreflection gratings, the morphology of the reflection grating differssignificantly. In particular, it has been determined that, unliketransmission gratings with similar liquid crystal concentrations, verylittle coalescence of individual droplets was evident. Furthermore, thedroplets that were present in the material were significantly smaller,having diameters between 50 and 100 nm. Furthermore, unlike transmissiongratings where the liquid crystal-rich regions typically comprise lessthan 40% of the grating, the liquid crystal-rich component of areflection grating is significantly larger. Due to the much smallerperiodicity associated with reflection gratings, i.e., a narrowergrating spacing (˜0.2 microns), it is believed that the time differencebetween completion of curing in high intensity versus low intensityregions is much smaller. Thus, gelation occurs more quickly and dropletgrowth is minimized. It is also believed that the fast polymerization,as evidenced by small droplet diameters, traps a significant percentageof the liquid-crystal in the matrix during gelation and precludes anysubstantial growth of large droplets or diffusion of small droplets intolarger domains.

Analysis of the reflection notch in the absorbance spectrum supports theconclusion that a periodic refractive index modulation is disposedthrough the thickness of the film. In PDLC materials that are formedwith the 488 nm line of an argon ion laser, the reflection notchtypically has a reflection wavelength at approximately 472 nm for normalincidence and a relatively narrow bandwidth. The small differencebetween the writing wavelength and the reflection wavelength(approximately 5%) indicates that shrinkage of the film is not asignificant problem. Moreover, it has been found that the performance ofsuch gratings is stable over periods of many months.

In addition to the materials utilized in the preferred embodimentdescribed above, it is believed that suitable PDLC materials could beprepared utilizing monomers such as triethyleneglycol diacrylate,trimethylolpropanetriacrylate, pentaerythritol triacrylate,pentaerythritol tetracrylate, pentaerythritol pentacrylate, and thelike. Similarly, other coinitiators such as triethylamine,triethanolamine, N,N-dimethyl-2,6-diisopropylaniline, and the like couldbe used instead of N-phenyl glycine. Where it is desirable to use the458 nm, 476 nm, 488 nm or 514 nm lines of an Argon lon laser, that thephotoinitiator dyes rose bengal sodium salt, eosin, eosin sodium salt,fluorescein sodium salt and the like will give favorable results. Wherethe 633 nm line is utilized, methylene blue will find ready application.Finally, it is believed that other liquid crystals, such as4′-pentyl-4-cyanobiphenyl or 4′-heptyl-4-cyanobiphenyl, can be utilizedin accordance with the invention.

Referring again to FIG. 8a, there is shown an elevational view of areflection grating 130 in accordance with the invention having periodicplanes of polymer channels 130 a and PDLC channels 130 b disposedparallel to the front surface 134 of the grating 130. The symmetry axis136 of the liquid crystal domains is formed in a direction perpendicularto the periodic channels 130 a and 130 b of the grating 130 andperpendicular to the front surface 134 of the grating 130. Thus, when anelectric field E is applied, as shown in FIG. 8b, the symmetry axis 136is already in a low energy state in alignment with the field E and willnot reorient. Thus, reflection gratings formed in accordance with theprocedure described above will not normally be switchable.

In general, a reflection grating tends to reflect a narrow wavelengthband, such that the grating can be used as a reflection filter. In thepreferred embodiment, however, the reflection grating is formed so thatit will be switchable. In accordance with the present invention,switchable reflection gratings can be made utilizing negative dielectricanisotropy LCs (or LCs with a low cross-over frequency), an appliedmagnetic field, an applied shear stress field, or slanted gratings.

It is known that liquid crystals having a negative dielectric anisotropy(Δ∈) will rotate in a direction perpendicular to an applied field. Asshown in FIG. 9a, the symmetry axis 136 of the liquid crystal domainsformed with a liquid crystal having a negative Δ∈ will also be disposedin a direction perpendicular to the periodic channels 130 a and 130 b ofthe grating 130 and to the front surface 134 of the grating. However,when an electric field E is applied across such gratings, as shown inFIG. 9b, the symmetry axis of the negative Δ∈ liquid crystal willdistort and reorient in a direction perpendicular to the field E, whichis perpendicular to the film and the periodic planes of the grating. Asa result, the reflection grating can be switched between a state whereit is reflective and a state where it is transmissive. The followingnegative Δ∈ liquid crystals and others are expected to find readyapplication in the methods and devices of the present invention:

Liquid crystals can be found in nature (or synthesized) with eitherpositive or negative Δ∈. Thus, in more detailed aspects of theinvention, it is possible to use a LC which has a positive Δ∈ at lowfrequencies, but becomes negative at high frequencies. The frequency (ofthe applied voltage) at which Δ∈ changes sign is called the cross-overfrequency. The cross-over frequency will vary with LC composition, andtypical values range from 1-10 kHz. Thus, by operating at the properfrequency, the reflection grating may be switched. In accordance withthe invention, it is expected that low crossover frequency materials canbe prepared from a combination of positive and negative dielectricanisotropy liquid crystals. A suitable positive dielectric liquidcrystal for use in such a combination contains four ring esters as shownbelow:

A strongly negative dielectric liquid crystal suitable for use in such acombination is made up of pyridazines as shown below:

Both liquid crystal materials are available from LaRoche & Co.,Switzerland. By varying the proportion of the positive and negativeliquid crystals in the combination, crossover frequencies from 1.4-2.3kHz are obtained at room temperature. Another combination suitable foruse in the present embodiment is a combination of the following:p-pentylphenyl-2-chloro-4-(p-pentylbenzoyloxy) benzoate and-4-(p-pentylbenzoyloxy) benzoate andp-heptylphenyl-2-chloro-4-(p-octylbenzoyloxy) benzoate. These materialsare available from Kodak Company.

In still more detailed aspects of the invention, is switchablereflection gratings can be formed using positive Δ∈ liquid crystals. Asshown in FIG. 10a, such gratings are formed by exposing the PDLCstarting material to a magnetic field during the curing process. Themagnetic field can be generated by the use of Helmholtz coils (as shownin FIG. 10a), the use of a permanent magnet, or other suitable means.Preferably, the magnetic field M is oriented parallel to the frontsurface of the glass plates (not shown) that are used to form thegrating 140. As a result, the symmetry axis 146 of the liquid crystalswill orient along the field while the mixture is fluid. Whenpolymerization is complete, the field may be removed and the alignmentof the symmetry axis of the liquid crystals will remain unchanged. (SeeFIG. 10b.) When an electric field is applied, as shown in FIG. 10c, thepositive Δ∈ liquid crystal will reorient in the direction of the field,which is perpendicular to the front surface of grating and to theperiodic channels of the grating.

FIG. 11a depicts a slanted transmission grating 148 and FIG. 11b depictsa slanted reflection grating 150. A holographic transmission grating isconsidered slanted if the direction of the grating vector G is notparallel to the grating surface. In a holographic reflection grating,the grating is said to be slanted if the grating vector G is notperpendicular to the grating surface. Slanted gratings have many of thesame uses as nonslanted gratings such as visual displays, mirrors, linefilters, optical switches, and the like.

Primarily, slanted holographic gratings are used to control thedirection of a diffracted beam. For example, in reflection holograms aslanted grating is used to separate the specular reflection of the filmfrom the diffracted beam. In a PDLC holographic grating, a slantedgrating has an even more useful advantage. The slant allows themodulation depth of the grating to be controlled by an electric fieldwhen using either tangential or homeotropic aligned liquid crystals.This is because the slant provides components of the electric field inthe directions both tangent and perpendicular to the grating vector. Inparticular, for the reflection grating, the LC domain symmetry axis willbe oriented along the grating vector G and can be switched to adirection perpendicular to the film plane by a longitudinally appliedfield E. This is the typical geometry for switching of the diffractionefficiency of a slanted reflection grating.

When recording slanted reflection gratings, it is desirable to place thesample between the hypotenuses of two right-angle glass prisms. Neutraldensity filters can then be placed in optical contact with the backfaces of the prisms using index matching fluids so as to frustrate backreflections which would cause spurious gratings to also be recorded. Theincident laser beam is split by a conventional beam splitter into twobeams which are then directed to the front faces of the prisms, and thenoverlapped in the sample at the desired angle. The beams thus enter thesample from opposite sides. This prism coupling technique permits thelight to enter the sample at greater angles. The slant of the resultinggrating is determined by the angle which the prism assembly is rotated(i.e., the angle between the direction of one incident beam and thenormal to the prism front face at which that beam enters the prism).

As shown in FIG. 12, switchable reflection gratings may be formed in thepresence of an applied shear stress field. In this method, a shearstress would be applied along the direction of a magnetic field M. Thiscould be accomplished, for example, by applying equal and oppositetensions to the two ITO coated glass plates which sandwich theprepolymer mixture while the polymer is still soft. This shear-stresswould distort the LC domains in the direction of the stress, and theresultant LC domain symmetry axis will be preferentially along thedirection of the stress, parallel to the PDLC planes and perpendicularto the direction of the applied electric field for switching.

Reflection gratings prepared in accordance with the teachings of thepresent invention will find application in color reflective displays,switchable wavelength filters for laser protection, and the like.

In another embodiment of the present invention, PDLC materials can bemade that exhibit a property known as form birefringence wherebypolarized light that is transmitted through the grating will have itspolarization modified. Such gratings are known as subwavelengthgratings, and they behave like a negative uniaxial crystal, such ascalcite, potassium dihydrogen phosphate, or lithium niobate, with anoptic axis perpendicular to the PDLC planes. Referring now to FIG. 13,there is shown an elevational view of a transmission grating 200 inaccordance with the present invention having periodic planes of polymerchannels 200 a and PDLC channels 200 b disposed perpendicular to thefront surface 204 of the grating 200. The optic axis 206 is disposedperpendicular to polymer planes 200 a and the PDLC planes 200 b. Eachpolymer plane 200 a has a thickness t_(P) and refractive index n_(p) andeach PDLC plane 200 b has a thickness t_(PDLC) and refractive indexn_(PDLC).

Where the combined thickness of the PDLC plane and the polymer plane issubstantially less than an optical wavelength (i.e.(t_(PDLC)+t_(P))<<λ), the grating will exhibit form birefringence. Asdiscussed below, the magnitude of the shift in polarization isproportional to the length of the grating. Thus, by carefully selectingthe length, L, of the subwavelength grating for a given wavelength oflight, one can rotate the plane of polarization or create circularlypolarized light. Consequently, such subwavelength gratings can bedesigned to act as a half-wave or quarter-wave plate, respectively.Thus, an advantage of this process is that the birefringence of thematerial may be controlled by simple design parameters and optimized toa particular wavelength, rather than relying on the given birefringenceof any material at that wavelength.

To form a half-wave plate, the retardance of the subwavelength gratingmust be equal to one-half of a wavelength, i.e. retardance=λ/2, and toform a quarter-wave plate, the retardance must be equal to one-quarterof a wavelength, i.e. retardance=λ/4. It is known that the retardance isrelated to the net birefringence, |Δn|, which is the difference betweenthe ordinary index of refraction, n_(o), and the extraordinary index ofrefraction n_(e), of the sub-wavelength grating by the followingrelation:

Retardance=|Δn|L=|n _(e) −n _(o) |L

Thus, for a half-wave plate, i.e. a retardance equal to one-half of awavelength, the length of the subwavelength grating should be selectedso that:

L=λ/(2|Δn|)

Similarly, for a quarter-wave plate, i.e. a retardance equal toone-quarter of a wavelength, the length of the subwavelength gratingshould be selected so that:

L=λ/(4|Δn|)

If, for example, the polarization of the incident light is at an angleof 45° with respect to the optic axis 210 of a half-wave plate 212, asshown in FIG. 14a, the plane polarization will be preserved, but thepolarization of the wave exiting the plate will be shifted by 90°. Thus,referring now to FIGS. 14b and 14 c, where the half-wave plate 212 isplaced between cross polarizers 214 and 216, the incident light will betransmitted. If an appropriate switching voltage is applied, as shown inFIG. 14d, the polarization of the light is not rotated and the lightwill be blocked by the second polarizer.

For a quarter wave plate plane polarized light is converted tocircularly polarized light. Thus, referring now to FIG. 15a, wherequarter wave plate 217 is placed between a polarizing beam splitter 218and a mirror 219, the reflected light will be reflected by the beamsplitter 218. If an appropriate switching voltage is applied, as shownin FIG. 15b, the reflected light will pass through the beam splitter andbe retroreflected on the incident beam.

Referring now to FIG. 16a, there is shown an elevational view of asubwavelength grating 230 recorded in accordance with theabove-described methods and having periodic planes of polymer channels230 a and PDLC channels 230 b disposed perpendicular to the frontsurface 234 of grating 230. As shown FIG. 16a, the symmetry axis 232 ofthe liquid crystal domains is disposed in a direction parallel to thefront surface 234 of the grating and perpendicular to the periodicchannels 230 a and b of the grating 230. Thus, when an electric field Eis applied across the grating, as shown in FIG. 15b, the symmetry axis232 distorts and reorients in a direction along the field E, which isperpendicular to the front surface 234 of the grating and parallel tothe periodic channels 230 a and 230 b of the grating 230. As a result,subwavelength grating 230 can be switched between a state where itchanges the polarization of the incident radiation and a state in whichit does not. Without wishing to be bound by any theory, it is currentlybelieved that the direction of the liquid crystal domain symmetry 232 isdue to a surface tension gradient which occurs as a result of theanisotropic diffusion of monomer and liquid crystal during recording ofthe grating and that this gradient causes the liquid crystal domainsymmetry to orient in a direction perpendicular to the periodic planes.

As discussed is Born and Wolf, Principles of Optics, 5th Ed., New York(1975) and incorporated herein by reference, the birefringence of asubwavelength grating is given by the following relation:${n_{e}^{2} - n_{o}^{2}} = {- \frac{\lbrack {( f_{PDLC} )\quad ( f_{P} )\quad ( {n_{PDLC}^{2} - n_{P}^{2}} )} \rbrack}{\lbrack {{f_{PDLC}\quad n_{PDLC}^{2}} + {f_{P}\quad n_{P}^{2}}} \rbrack}}$

wheren_(o) = the  ordinary  index  of  refraction  of  the  subwavelength  grating;n_(e) = the  extraordinary  index  of  refraction;n_(PDLC) = the  refractive  index  of  the  PDLC  plane;n_(P) = the  refractive  index  of  the  polymer  plane;$n_{LC} = \begin{matrix}{{{the}\quad {effective}\quad {refractive}\quad {index}\quad {of}\quad {the}\quad {liquid}\quad {crystal}\quad {seen}\quad {by}\quad {an}}{\quad \quad}} \\{{{incident}\quad {optical}\quad {wave}};}\end{matrix}$ f_(PDLC) = t_(PDLC)/(t_(PDLC) + t_(P))f_(P) = t_(P)/(t_(PDLC) + t_(P))

Thus, the net birefringence of the subwavelength grating will be zero ifn_(PDLC)=n_(P).

It is known that the effective refractive index of the liquid crystal,n_(LC), is a function of the applied electric field, having a maximumwhen the field is zero and a value equal to that of the polymer, n_(P),at some value of the electric field, E_(MAX). Thus, by application of anelectric field, the refractive index of the liquid crystal, n_(LC), and,hence, the refractive index of the PDLC plane can be altered. Using therelationship set forth above, the net birefringence of a subwavelengthgrating will be a minimum when now is equal to no, i.e. whenn_(LC)=n_(P). Therefore, if the refractive index of the PDLC plane canbe matched to the refractive index of the polymer plane, i.e.n_(PDLC)=n_(P), by the application of an electric field, thebirefringence of the subwavelength grating can be switched off.

The following equation for net birefringence, i.e. |Δn|=|n_(e)−n_(o)|,follows from the equation given in Born and Wolf (reproduced above):${\Delta \quad n} = {- \frac{\lbrack {( f_{PDLC} )\quad ( f_{P} )\quad ( {n_{PDLC}^{2} - n_{P}^{2}} )} \rbrack}{\lbrack {2{n_{AVG}( {{f_{PDLC}\quad n_{PDLC}^{2}} + {f_{P}\quad n_{P}^{2)}}} \rbrack}} }}$

where n_(AVG)=(n_(e)+n_(o))/2

Furthermore, it is known that the refractive index of the PDLC planen_(PDLC) is related to the effective refractive index of the liquidcrystal seen by an incident optical wave, n_(LC), and the refractiveindex of the surrounding polymer plane, n_(P), by the followingrelation:

n _(PDLC) =n _(P) +f _(LC) [n _(LC) −n _(P)]

where f_(LC) is the volume fraction of liquid crystal dispersed in thepolymer within the PDLC plane, f_(LC)=[V_(LC)/(V_(LC)+V_(P))].

By way of example, a typical value for the effective refractive indexfor the liquid crystal in the absence of an electric field isn_(LC)=1.7, and for the polymer layer n_(P),=1.5. For a grating wherethe thickness of the PDLC planes and the polymer planes are equal (i.e.t_(PDLC)=t_(P),f_(PDLC)=0.5 f_(P)) and f_(LC)=0.35, the netbirefringence, Δn, of the subwavelength grating is approximately 0.008.Thus, where the incident light has a wavelength of 0.8 μm, the length ofthe subwavelength grating should be 50 μm for a half-wave plate and 25μm for a quarter-wave plate. Furthermore, by application of an electricfield of approximately 5 V/μm, the refractive index of the liquidcrystal can be matched to the refractive index of the polymer and thebirefringence of the subwavelength grating turned off. Thus, theswitching voltage, V_(n), for a half-wave plate is on the order of 250volts, and for a quarter-wave plate approximately 125 volts.

By applying such voltages, the plates can be switched between the on andoff (zero retardance) states on the order of microseconds. As a means ofcomparison, current Pockels cell technology can be switched innanoseconds with voltages of approximately 1000-2000 volts, and bulknematic liquid crystals can be switched on the order of millisecondswith voltages of approximately 5 volts.

In an alternative embodiment of the invention shown in FIG. 17, theswitching voltage of the subwavelength grating can be reduced bystacking several subwavelength gratings 220 a-e together, and connectingthem electrically in parallel. By way of example, it has been found thata stack of five gratings each with a length of 10 μm yields thethickness required for a half-wave plate. It should be noted that thelength of the sample is somewhat greater than 50 μm, because eachgrating includes an indium-tin-oxide coating which acts as a transparentelectrode. The switching voltage for such a stack of plates, however, isonly 50 volts.

Subwavelength gratings in accordance with the present invention areexpected to find suitable application in the areas of polarizationoptics and optical switches for displays and laser optics, as well astunable filters for telecommunications, colorimetry, spectroscopy, laserprotection, and the like. Similarly, electrically switchabletransmission gratings have many applications for which beams of lightmust be deflected or holographic images switched. Among theseapplications are: Fiber optic switches, reprogrammable N×N opticalinterconnects for optical computing, beam steering for laser surgery,beam steering for laser radar, holographic image storage and retrieval,digital zoom optics (switchable holographic lenses), graphic arts andentertainment, and the like.

A switchable hologram is one for which the diffraction efficiency of thehologram may be modulated by the application of an electric field, andcan be switched from a fully on state (high diffraction efficiency) to afully off state (low or zero diffraction efficiency). A static hologramis one whose properties remain fixed independent of an applied field. Inaccordance with the present invention, a high contrast static hologramcan also be created. In this variation of the present invention, theholograms are recorded as described previously. The cured polymer filmis then soaked in a suitable solvent at room temperature for a shortduration and finally dried. For the liquid crystal E7, methanol hasshown satisfactory application. Other potential solvents includealcohols such as ethanol, hydrocarbons such as hexane and heptane, andthe like. When the material is dried, a high contrast static hologramwith high diffraction efficiency results. The high diffractionefficiency is a consequence of the large index modulation in the film(Δn˜0.5) because the second phase domains are replaced with empty (air)voids (n˜1).

Similarly, in accordance with the present invention, a highbirefringence static sub-wavelength wave-plate can also be formed. Dueto the fact that the refractive index for air is significantly lowerthan for most liquid crystals, the corresponding thickness of thehalf-wave plate would be reduced accordingly. Synthesized wave-plates inaccordance with the present invention can be used in many applicationsemploying polarization optics, particularly where a material of theappropriate birefringence at the appropriate wavelength is unavailable,too costly, or too bulky.

In the claims, the term polymer-dispersed liquid crystals andpolymer-dispersed liquid crystal material includes, as may beappropriate, solutions in which none of the monomers have yetpolymerized or cured, solutions in which some polymerization hasoccurred, and solutions which have undergone complete polymerization.Those of skill in the art in the field of the invention will clearlyunderstand that the use herein of the standard term used in the art,polymer-dispersed liquid crystals (which grammatically refers to liquidcrystals dispersed in a fully polymerized matrix) is meant to includeall or part of a more grammatically correct prepolymer-dispersed liquidcrystal material or a more grammatically correct starting material for apolymer-dispersed liquid crystal material.

It will be seen that modifications to the invention as described may bemade, as might occur to one with skill in the field of the invention,within the intended scope of the claims. Therefore, all embodimentscontemplated have not been shown in complete detail. Other embodimentsmay be developed without departing from the spirit of the invention orfrom the scope of the claims.

What is claimed is:
 1. A hologram made by exposing an interferencepattern inside a polymer-dispersed liquid crystal material, thepolymer-dispersed liquid crystal material comprising, before exposure:(a) a polymerizable monomer; (b) vinyl neononanoate; (c) a liquidcrystal; (d) a coinitiator; and (e) a photoinitiator dye.
 2. Thehologram of claim 1, wherein the polymer-dispersed liquid crystalmaterial comprises, before exposure, about 6.5% by weight vinylneononanoate.
 3. The hologram of claim 1, wherein the polymerizablemonomer comprises dipentaerythritol hydroxypentaacrylate.